Mozilla/5.0 is the general token that says the browser is Mozilla compatible, and is common to almost every browser today.

platform describes the native platform the browser is running on (e.g. Windows, Mac, Linux or Android), and whether or not it's a mobile phone. Firefox OS phones simply say "Mobile"; the web is the platform. Note that platform can consist of multiple "; "-separated tokens. See below for further details and examples.

rv:geckoversion indicates the release version of Gecko (such as "17.0"). In recent browsers, geckoversion is the same as firefoxversion.

Gecko/geckotrail indicates that the browser is based on Gecko.

On Desktop, geckotrail is the fixed string "20100101"

Firefox/firefoxversion indicates the browser is Firefox, and provides the version (such as "17.0").